Solid state image pickup devices comprising a charge transfer device such as a charge coupled device ( hereinafter referred to as a CCD ) are classified broadly into the frame transfer type and the interline transfer type, and each of them is utilized in different way so as to make a good use of its advantages and features. One example of the solid state image pickup device of the interline transfer type using the CCD comprises a photo-sensing and vertical transfer portion which includes a plurality of photodetectors arranged in horizontal and vertical rows and vertical transfer portions each formed with a group of CCDs and extending along each of the vertical rows of the photodetectors, a horizontal transfer portion coupled with the photo-sensing and vertical transfer portion and an output portion coupled with the horizontal transfer portion and provided with an signal output terminal, the whole of which are formed on a common semiconductor substrate.
In a solid state image pickup apparatus employing such a solid state image pickup device of the interline transfer type using the CCD, a vertical driving signal for vertical charge transfer is applied to the vertical transfer portions through vertical transfer electrodes which are provided on an insulating layer overspreading the photosensing and vertical transfer portion and a horizontal driving signal for horizontal charge transfer is applied to the horizontal transfer portion through horizontal transfer electrodes which are provided on an insulating layer overspreading the horizontal transfer portion, so that vertical charge transfer operation and horizontal charge transfer operation are performed in the vertical transfer portions and the horizontal transfer portion, respectively. For example, signal charges obtained in the photodetectors in response to the light received thereby during one frame period are read out to the vertical transfer portions, and then transferred toward the horizontal transfer portion by the vertical charge transfer operation performed in the vertical transfer portion so that the signal charges produced in each horizontal row of the photodetectors are transferred in turn to the horizontal transfer portion. The signal charges transferred to the horizontal transfer portion are further transferred to the output portion by the horizontal charge transfer operation performed in the horizontal transfer portion and as a result of this an image pickup signal output is obtained at the signal output terminal.
When a picture is reproduced in response to an image pickup signal output obtained under such operation of the solid state image pickup device of the interline transfer type as described above in a previousely proposed solid state image pickup apparutus, it is often seen that such a defect as to appear to be a pair of black and white spots contiguous up and down to each other arises on the reproduced picture. This defect is usually called "white and black spots", and the black spot therein tends to elongate when the image pickup signal output is produced under relatively low illuminance. Such white and black spots can not be eliminated by means of ordinary defect correction, because they result from a local area of deep ( high ) potential residing in the vertical transfer portion. This local area of deep potential in the vertical transfer portion appears at a region having high impurity density which is provided undesirably due to nonuniformity in impurity density in the vertical transfer portion or at a region positioned under a thick portion of the insulating layer which is provided undesirably due to nonuniformity in thickness of the insulating layer on the vertical transfer portion, and is a structural defect in the image pickup device.
In the case of the image pickup device in which such a local area of deep potential in the vertical transfer portion as described above resides, an insulated gate field effect transistor ( hereinafter referred to as a MOS FET ) having its source at the local area of deep potential is substantially formed in the vertical transfer portion and a flow of charges is caused from the local area of deep potential to an advanced potential well in the vertical transfer portion as a sub-threshold current of the MOS FET. If a charge transfer period in which each charge transfer in the vertical transfer portions is carried out is constant and the manner of repetition of such charge transfer periods is made uniform, the amount of charges flowing into the advanced potential well from the local area of deep potential is not varied and therefore the charges flowing into the advanced potential well from the local area of deep potential do not raise any serious problem. However, as to the charge transfer operation in the vertical transfer portions, an exceptional situation in which the charge transfer period is elongated in comparison with that in the normal situation arises, for example, once a frame period, and during such elongated charge transfer period, the local area of deep potential sends the charges stored therein in large quantities into the advanced potential well so as to supply excessive signal charges to the latter and, to the contrary, to make areas positioned behind be lacking in signal charges. This results in that the signal charges are in unbalanced quantities so that an image pickup signal output developed therefrom is deteriorated and the white and black spots appear on a picture reproduced in response to the deteriorated image pickup signal output.
In view of the cause of the white and black spots mentioned above, it is understood that the problem of the white and black spots can be solved by means of maintaining the charge transfer period, in which each charge transfer in the vertical transfer portions is carried out, to be constant regardless of the operation for reading out the signal charges to the vertical transfer portions from the photodetectors.
Accordingly, it is an object of the present invention to provide an solid state image pickup apparatus employing a solid state image pickup device of the interline transfer type, in which the driving condition for charge transfer performed in vertical transfer portions provided in the solid state image pickup device is improved so that a charge transfer period in which each charge transfer in the vertical transfer portions is carried out is made uniform and consequently an image pickup signal output from which a picture without the defect called usually "white and black spots" is reproduced can be obtained from the apparatus.